1. Field of the Invention
The present invention relates to a method and an apparatus suitable for forming a thick film having a thickness, for example, of 20 μm or more on a surface of a substrate such as a glass substrate, a semiconductor wafer or the like, and also relates to a tray for use in a film-forming process.
2. Description of the Prior Art
Conventionally, for forming a resist film or a SOG film on a substrate such as a glass substrate, a semiconductor wafer or the like, a spin coating method and a slit nozzle method have been known.
In the spin coating method, a plate-like material to be treated is spun by a spinner at a high rate, and a coating liquid, which drops at the center of the plate-like material, is dispersed by a centrifugal force. In a method, such as disclosed in Japanese Unexamined Patent Application Publication No. Hei 3-56163, a concave portion is defined on the upper surface of the spinner head to accommodate a plate-like material to be treated.
In the slit nozzle method such as disclosed in U.S. Pat. No. 4,696,885, a coating liquid is discharged from a slit nozzle having almost the same width as a plate-like material to be treated, and the coating liquid is applied on a predetermined area by moving the plate-like material with respect to the slit nozzle.
Recently, trials of forming a thick film on a surface of a substrate have been performed. For example, it is necessary to form a resist film having a thickness of around 20 μm so as to form a protruding electrode, which is referred to as a bump, having a height of around 20 μm on a surface of an IC pattern by applying integrated circuit forming technology.
In addition, wire bonding, which has conventionally been used for mounting an IC chip on a substrate, requires labor and time because it is necessary to connect metal wires one by one in wire bonding. Thus, instead of wire bonding, there is another way, in which a plurality of metal posts are provided on a chip, and the chip is mounted on a substrate via the posts. Since the metal posts have a height of around 100 μm, it is necessary to form a resist film having a thickness of around 100 μm so as to form metal posts by applying integrated circuit forming technology.
However, in the spin coating method, only 5% or so of the supplied coating liquid can be used to form a film, and a large part of the coating liquid is scattered away by the centrifugal force. Thus, a thick film cannot be formed, which is a drawback.
In addition, there is a tendency that the size of a substrate (glass substrate) is increased and the thickness is reduced. Therefore, there is another drawback that the substrate is undesirably flexed due to its weight at the time of transfer or the like.
On the other hand, in the slit nozzle method, the consumption of a coating liquid can be reduced and the thickness of a film can be increased. However, this method has a drawback that the film thickness becomes extremely greater at the point where coating is started and the point where the coating is ended compared to the other points because of the surface tension.
Moreover, the above-mentioned prior art materials relate to processes up to applying a coating liquid. Specifically, the prior art materials are not directed to improvement of the efficiency in a process of heating and drying a coating liquid after being applied and the subsequent processes.
In particular, in a case of forming a thick film, there is a problem unique to a conventional apparatus because the size of a substrate becomes large nowadays. Specifically, at the time of heating and drying a coating liquid applied to a substrate, the surface of the coating liquid is dried and cured earlier than the inside of the coating liquid. As a result, a solvent contained in the inside of the coating liquid cannot be discharged, and the residual ratio of a solvent becomes different depending on the area of the coating liquid. This causes a problem that a pattern having a high aspect ratio is stripped from the surface of a substrate after development and/or wrinkling is generated in the coating film.
Also, even if it is possible to prevent a substrate from being flexed in the course of processes by transferring the substrate in a state of being mounted on a tray, there is a strong likelihood that the substrate will be broken because of its weight when the substrate is lifted from the tray in a case of forming a thick film on the substrate.